1. Field of the Invention
The present invention relates to a method for controlling vehicle transmission systems, and more particularly to a control method for a vehicle transmission system which includes a friction type clutch and a gear type transmission and is electronically controlled in accordance with the operation condition of the associated vehicle to perform automatic gear-shift operations.
2. Description of the Prior Art
Japanese Patent Application Public Disclosure No. Sho 63-115955 (115955/88) discloses a vehicle automatic transmission system of the type described above, which has a control mode in which low speed vehicle running can be established within a low gear range with the clutch engaged without need to depress the accelerator pedal, which will be referred to as idle running, and another control mode in which the clutch is disengaged when the amount of operation of the accelerator pedal becomes less than a prescribed level, as in a vehicle drive-off mode for snow-covered road condition. However, the vehicle transmission system of the type described above can be operated in still another control mode in which the clutch is engaged for obtaining the braking power of the engine when the vehicle speed increases from a low vehicle speed which is assumed to be tantamount to a vehicle halt condition. This causes certain disadvantages. For example, in the case where the automatic transmission system is operating in a mode in which idle running is impossible, such as the vehicle drive-off mode for snow-covered road condition, when the vehicle is traveling downhill, the clutch may once be engaged to obtain the braking power of the engine but will be automatically disengaged again when the vehicle running speed decreases under the braking power of the engine. Furthermore, after this, as the vehicle accelerates to a high speed by the downhill coasting, the clutch is automatically engaged again to provide the braking power of the engine. Thus, the engaging/disengaging operation of the clutch will be alternatively carried out irrespective of the driver's intention and the accelerating operation and the braking operation are apt to be effected alternatively. This degrades the feel of the vehicle operation.